Wave translation system



March 3 1935. A. L. STILLWELL WAVE TRANSLATION SYSTEM Filed Jan. 7,193:;

LINE 1 2- F IG.

AMPLIFIER BRIDGE LINE 122mm: or

AMPLIFIER EOUALIZEI? LOSS I LOSS /l- "E USEFUL o kUXQQH-uk FREQUENCY INKILOCYCLES INVENTOR ALST/LLWELL Patented Mar. 12 1935 STATES PATENT OFFICE I 1,993,758 v, 1 1 1 WAVE TRANSLATION SYSTEM Albert L. Still welI,Westfield, N. J., assignor to Bell Telephone Laboratories, Incorporated,New York, 1L, a corporation of New York 7 Application January 1932,Serial No. 585,216 7 Claims. (Cl. 178-44) This invention relates" toamplifying systems and more particularly to arrangements for controllingthe frequency variation of the gain of an amplifier.

"6 An object of the invention is toobtain a desired gain-frequencycharacteristic in an amplifier or repeater in a. transmission system inwhich the transmission of currents of a wide range of frequencies isrequired. For example the amplification characteristic may be such as tocompensate the attenuation distortion of a telephone line throughout therange of speech or carrier frequencies.

This is accomplished in accordance with the invention by the use of aretroactive connection in the amplifier including a network in the amplifier output circuit which controls the frequency variation of theretroa'ctiveeifect; A feature of the invention is the arrangement of thefeed-back controlling network whereby, in addition to providing adesired retroaction characteristic, it serves to prevent the transfer ofenergyjfrom the connected load to the amplifier input, without theintroduction of any substantial transmission loss, and to maintain theproper impedance relationship between the amplifier and its outputcircuits.

The invention will be described in detail hereinafter with reference'tothe accompanying drawing'inwhichz K Fig. 1 is a simplified schematic ofthe system ofthe invention;

Fig. 2 is a detailed schematic of the system shown diagrammatically inFig. 1;

Figs. 3, 4 and 5 are diagrams useful in explaining the various aspectsof the invention; and

Fig. 6'shows transmission characteristics obtainable with the network ofthe invention.

In the system shown in Fig. l an amplifier 10 is inserted between twoline sections 11 and 12. A bridge equalizer network 13 is used inconnecting the output circuit of the amplifier 10 with the outgoing linesection 12. The connections ofthe bridge 13 aresuch that the amplifiercan transmit with small attenuation into the line 12 and also through afeed-back circuit 14 to a Wheatstone bridge 15 in the amplifier input.The bridge network 13 is balanced'so that the line 12 is substantiallyisolated from the feed-back circuit, and bridge 15 is likewise balancedto prevent the retroaction currents entering line 11 whilepermittingthese currents to be transmitted to the amplifier input.

' Details of the arrangementoi Fig. 1 are shown in Fig. 2 in which theamplifier locomprises a vacuum tube 17 with associatedpow'er supply "emcuits. The incoming line 11 is connected to the' input circuit of tube17 through the input bridge 15 and the output circuit of the tube 1'7is'connected to the outgoing line 12 through the bridge equalizer 13.The input bridge 15 comprises four resistance arms R1, R2. R3 and R4arranged in the usual manner, the four junction points being designatedA B,C' and D. Theinput circuit of the tube 17 is connected between theterminals A and B while the line 11 is connected between Y B and D. Thefeed-back circuitfrom the output bridge-equalizer 13 is connectedbetween Af I and C in conjugate relation to the line 11. The bridgeequalizer 13 is also composed offour'arms R, Z11, Z21 and R0, the latterbeing represented by the internal impedance of the tube 17. The cornersof this bridge are designated A, B, C and D. The tube impedance R0 isconnected between A'and B; Z11is connected between Band C; R is betweenC and D; and Z21 is between D and' A".

The outgoing line 12 is connected'b'etween- B and D'and the feed-backconnection is made toterminals A and 0. As shown diagrammatically inFig.3 the line 12 and thefeed-back'circuit, which includes resistancebridge 15, form the diagonal branches of bridge 13'and may be madeconjugate by balancing the bridge. Balancing of the bridge network isaccomplished by making" the resistance branches equaLthat isR'=Ro,*andby con structing Zn and Z21 as inverse imp in accordance The;network of'Fig; 3 without changein' any 3 with the formula v Z11Z21=R0edances related part presents other aspects thatmay be brought intoprominence by redrawing thev diagram as shown respectively by Figs. 4and 5. Fig. 4 em phasizes the transmission relation between theamplifier space path resistance R0, and the ime pedance of line 12.. Theimpedances Z11 and Z21 together with resistance R and the resistance ofthe feed-back circuit constitute a bridged-T network, which, when thebranch impedances are so proportioned that the resistance of thefeedbackcircuit and the impedance of line12 are both equal to R0, becomes aconstant resistancegnete work of the type'disclosed in U. S. Patent1,606,817

issued November 16, 1926, to G. I-I..Stev enson.

.W kr

The propagati given by The loss repre the line loss.

on constant, P1 of this net In order that the loss between the amplifierand line 12 may be small it is desirable that the impedance Z11 shouldbe large. The condition that the feed-back circuit resistance be equalto R is achieved by so proportioning the resistances of bridge 15 thatin addition to conforming to the condition of balance, namelyR1'Rs'=R2"R4',

they are related in accordance with the equation Fig. 5 emphasizes thetransmission relation between the space path of the amplifier and thefeedback circuit, which latter as above mentioned includes bridge 15. Inthis aspect the coupling network constituted by the impedance of line12, resistance R, and impedances Z11 and Z21 also forms a constantresistance bridged-T network of the type shown in the above mentionedpatent to Stevenson. The voltage transmitted through this circuit isimpressed across terminals A and C of bridge 15 and a constant fractionof this voltage represented by the ratio is impressed upon the grid oftube 1'7. This voltage is in substantially opposite phase to the voltageimpressed on the grid from line 11 and serves to diminish theamplification of the system.

The frequency variation of the voltage fed back through this circuit isdetermined by the propagation constant P2 of the coupling network ofFig. 5, the value of P2 being given by The loss represented by thispropagation constant will be designated as the equalizer loss.

As already indicated maintenance of a low line loss requires that theimpedance Z11 shall be large and that impedance Z21 be correspondinglysmall. This requirement corresponds to a large loss in the. circuitrepresented by Fig. 5 and hence to a relatively small feed-back voltage.The impedance- Z11, however, may be strongly variable with frequency,although always large enough to main.- tain a low line loss, and due tothis variation the value of the feed-back voltage and hence of theeffective amplification may be made to vary through a wide range for agiven frequency change.

The particular form of the bridge 13 illustrated in Fig. 2 is suitablefor the equalization of the loss in a transmission line at carriercurrent frequencies, for example line 11, which increases at asubstantially uniform rate with frequency. In this case it is necessarythat the amplification gain should increase with increasing frequencyand, therefore, that the degenerative: retroactive effect should bestrongest at the lower frequencies. This requires that the impedance Z11should increase in value with frequency. As illustrated Z11 comprises ananti-resonant circuit in parallel with a series combination of aresistance and an inductance, the anti-resonant circuit being tuned to afrequency somewhat higher than the upper limitingfrequency of thedesired operating range. The impedance Z21 is of course of inverse typeto Z11.

The frequency variation of the line loss and the equalizer loss in thiscase is illustrated by the curves of Fig. 6.v The inverse relationshipbetween these losses indicated by Equations 2 and 4 small.

is demonstrated by the form of these curves. It will be noted that theline loss, while negligibly small at the higher frequencies becomesconsiderable at very low frequencies. The extent to which it increasesmay be controlled by proper design of impedances Z11 and Z21, an inverseeffect being produced in the low frequency value of the equalizer loss.The useful frequency range, as indicated in the diagram is from 4 to 40kilocycles per second and in this range the line loss is quite Thefrequency variation of both losses may be so coordinated by the designof impedances Z11 and Z21 so that the net over-all amplification of thesystem accurately compensates the attenuation distortion of theassociated line. The increasing value of the line loss at frequenciesbelow the useful range may be used to advantage under certaincircumstances, for example in a fourwire transmission circuit in which anumber of amplifiers such as shown in Fig. 2 is included. The line lossin this case will serve to prevent low frequency singing around thefourwire loop.

What is claimed is:

l. A transmission system comprising a line having a frequency selectivetransmission characteristic, an amplifier in said line, input and outputcircuits for said amplifier, a feed-back circuit for connecting saidoutput circuit to said input circuit and a unitary network adapted tosubstantially prevent transmission from said line to said feed-backcircuit or vice versa and having a frequency selective transmissioncharacteristic for currents transmitted from said output circuit to'said feed-back circuit, which simulates the transmission characteristicof the line.

2. A transmission system comprising a line having a frequency selectivetransmission characteristic, an amplifier in said line, said amplifierrequiring for its eflicient operation the equalization of said linecharacteristic, input and output circuits for said amplifier, afeed-back circuit for connecting said output circuit to said inputcircuit and a unitary network adapted to equalize said linecharacteristic both by selectively attenuating currents passing fromsaid output circuit to said feed-back circuit in a manner to simulatesaid line and by selectively transmitting currents from said outputcircuit to said line in a manner adapted to off-set the transmissioncharacteristic of the line.

3. A transmission system comprising a. line having a frequency selectivetransmission characteristic, an amplifier in said line, said amplifierrequiring for its efiicient operation the equalization of said linecharacteristic, inputand output circuits for said amplifier, a feed-backcircuit for connecting said output circuit to said input circuit and aunitary network adapted to equalize said line characteristic both byselectively attenuating currents passing from said output circuit tosaid feed-back circuit in a manner to simulate said line and byselectively transmitting currents from said output circuit to said linein a manner adapted to off-set the transmission characteristic of theline, said network substantially preventing transmission from saidlineto said feed-back circuit or vice versa.

4. A transmission system comprising a line having a frequency selectivetransmission characteristic, an amplifier in said line, said amplifierrequiring for its eflicient operation the equalization of said linecharacteristic, input and output circuits for said amplifier, afeed-backcircuit for connecting said output circuit to said inputcircuit and a unitary network adapted to equalize said linecharacteristic both by selectively attenuating currents passing fromsaid output circuit to said feed-back circuit in a manner to simulatesaid line and by selectively transmitting currents from said outputcircuit to said line in a manner adapted to off-set the transmissioncharacteristic of the line over a range of useful frequencies, saidnetwork greatly attenuating frequencies below said range to preventsinging.

5. In a system for controlling transmission, an amplifier, a lineassociated therewith, a feed-back circuit connected to said amplifierand a bridge network comprising a pair of inverse impedances, a pair ofresistance arms connected alternately beiwveen said inverse impedances,said line being connected across one pair of diagonally opposite cornersof said bridge, said feed-back circuit being connected to said bridge inconjugate relation to said line, and said amplifier constituting one ofsaid resistance arms.

6. In a transmission system, a section of transmitting medium having avarying efiiciency dependent upon the frequency of the wave transmittedtherethrough, an amplifier for off-setting at least a portion of thetransmission loss of said section, input and output circuits for saidamplifier, a feed-back circuit for connecting said output circuit withsaid input circuit, incoming and outgoing lines for connecting saidamplifier to the medium, a network comprising a resistance and twogeneral impedances, said resistance and one of said general impedancesbeing connected across one of said lines, said feed-back circuit beingconnected across the resistance and the other of said generalimpedances, and one of the said circuits of the amplifier beingconnected across the entire network, said network being designed tosubstantially prevent transmission between said line and feed-backcircuit, to approximately simulate the varying efliciency of saidsection to be equalized for transmission between said amplifier and saidfeed-back circuit and to cause a relatively small transmission lossbetween said'amplifier and said line.

7. In a transmission system, a transmitting medium having a varyingefliciency dependent upon the frequency of the wave transmittedtherethrough, an amplifier for oif-setting at least a portion of thetransmission loss of said medium, input and output circuits for saidamplifier, a feed-back circuit for connecting said output circuit withsaid input circuit, incoming and outgoing lines for connecting saidamplifier into said medium, a network comprising a resistance and twogeneral impedances, said resistance and one of said impedances beingbridged between the terminals of one of said lines, said feed-backclr-'- cuit being bridged across said resistance and the other of saidgeneral impedances, and one of said amplifier circuits being bridgedacross the whole while substantially preventing transmission betweensaid line and said feed-back circuit.

ALBERT L. STILLWELL,

